An RTD (Resistance Temperature Detector) measures temperature by exploiting the predictable increase in electrical resistance of a metal element as temperature rises. PT100 — the most common industrial type — uses a platinum element with a resistance of exactly 100 Ω at 0 °C. At 100 °C the resistance is approximately 138.5 Ω; the relationship is defined by the IEC 60751 standard (the α = 0.00385 Ω/Ω/°C coefficient curve).
**RTD vs Thermocouple — the core trade-off:**
- **Accuracy:** PT100 offers ±0.15 °C (Class AA) to ±0.5 °C (Class B) accuracy. A Type K thermocouple is typically ±1.5 °C to ±2.5 °C. For processes where 1 °C matters, the RTD wins decisively.
- **Range:** Thermocouples cover -200 °C to +1260 °C (Type K). PT100 covers -200 °C to +850 °C. For temperatures above 600 °C (furnaces, kilns, exhaust gas), a thermocouple is the only option.
- **Linearity:** Platinum resistance vs temperature is nearly linear over the industrial range. Thermocouple output is nonlinear and requires a polynomial correction table in the module.
- **Stability:** PT100 sensors drift less than 0.1 °C per year at moderate temperatures. High-temperature thermocouples can drift several degrees per year.
- **Cost:** RTD sensors and their input modules cost more than thermocouple equivalents. For high-density temperature monitoring (10+ points), the cost differential becomes significant.
**2-wire, 3-wire, and 4-wire configurations** exist because the lead resistance of the cable between the RTD and the PLC module adds to the measured resistance, introducing a temperature error:
- **2-wire:** the cable resistance is included in the measurement. Accurate only for very short cable runs (<1 m) or when cable resistance is calibrated out. Avoid in industrial settings.
- **3-wire:** the most common industrial configuration. A third conductor allows the module to measure and subtract the lead resistance. Eliminates most lead-resistance error assuming the three conductors have equal resistance (same gauge, same length, same temperature).
- **4-wire (Kelvin):** true four-wire measurement. Two wires carry the excitation current; two separate wires measure the voltage drop across the element only. Lead resistance has zero effect. Used in precision laboratory and pharmaceutical applications. Requires a 4-wire RTD input channel.
**PLC wiring:** RTD input modules (e.g. Siemens SM331 RTD, Allen-Bradley 1756-IR6I) supply a precise excitation current to the RTD and measure the resulting voltage. The module applies the IEC 60751 linearisation and outputs a scaled integer — typically 0 to 27648 representing 0.0 °C to a configurable upper range. Shielded cable, grounded at the panel end only, is mandatory for accurate low-resistance measurement.
